Flexible coated abrasive articles generally comprise a flexible backing material, usually made of woven or nonwoven fibers or paper or a combination of these materials, one side of which is provided with adhesively anchored abrasive grits. When these articles are used in an endless belt machine to sand wood and plastic, and often when they are used to grind some metals, such as light metal alloys, voltages in the order of magnitude from 50 to 100 kilovolts may be generated. The voltage is generated where the endless belt travels between a back support and the workpiece. The high electrostatic voltage is of considerable inconvenience. A sudden discharge may take place and cause a serious accident by shocking the workman to a sudden, uncontrolled movement. The charge also causes the working space to become dusty and dirty due to electrostatic accumulation of dust on machines and workpieces, which, due to their electrical charge, are not removed by the usual suction means. If the dust is very fine, it may also be held in suspension in the air for a very long time, contaminating the entire working area and presenting an obvious health hazard as well.
Many common kinds of wood retain a residual charge for a very long time. It has proved nearly impossible in practice to use any dry process to remove this dust prior to painting, or the like. It is, of course, not very convenient to use wet process since the wood fibers then tend to "rise" and roughen the surface. It should be obvious from the above that a method for removing or preventing electrostatic charges in work of the above mentioned kind would be of great advantage.
One method of accomplishing this has been described in the copending application of Eric L. Markoo, Tore G. H. Strand and Thorsten W. Sandell, entitled "Abrasive Articles," Ser. No. 387,748, filed Aug. 13, 1973, and now U.S. Pat. No. 3,942,959 which proposes an electroconductive layer in the abrasive article, sandwiched between two non- or semi-conducting layers. Such an abrasive article has proved to be extremely effective, and even after prolonged use no electrostatic charge buildup has been measurable. It would thus appear that providing a continuous layer of electroconductive material in a conventional abrasive article would present a complete solution to the problem related above. However, flexible coated abrasive articles of this kind are manufactured in fully automatic plants, and not all articles need to be anti-electrostatic. There is a need, therefore, for a method of providing conventional abrasive articles with anti-electrostatic properties without having to change the standard manufacturing process.
It should be pointed out that although metallizing the abrasive article would seem to be an obvious expedient, it is, unfortunately, no useful way of solving the electrostatic problem. Probably due to heat reflection, a metallized abrasive belt may become extremely hot during dry-sanding of wood. In addition, it is not a good practice to metallize the abrasive side, since this may contaminate the workpiece with metal particles, which later may discolor the wood and/or react with lacquer or varnish or dye.